The present invention relates to positioning devices and, more particularly, to a positioning device having a floating shuttle propelled by independently controlled linear motors, capable of linear motion along two axes and rotary motion, and employing a closed loop control system.
Conventional positioning systems employ electric motors which drive lead screws oriented about orthogonal axes. A table is supported upon a set of rails, or their equivalent, and incorporates a recirculating ball nut which engages the lead screw and thereby propels the table upon rotation of the lead screw. Motion is thus provided along a single linear axis. To facilitate motion along two orthogonal axes, the aforementioned apparatus may be mounted normal to and upon a second set of rails further incorporating a lead screw to drive a recirculating ball nut propelling the first set of rails. The mass of the entire first apparatus must therefore be propelled by the second apparatus, limiting the speed of operation. The use of lead screws and ball nuts requires expensive components and time consuming alignment. Additionally, the mass of the components results in substantial inertia being developed and thus restricts the rapid acceleration and de-acceleration of the table. Furthermore, wear upon the lead screw, the recirculating balls, and the rails results in decreased accuracy, down-time, and maintenance costs.
Other conventional positioning systems employ linear motors which drive a table along orthogonal X-Y coordinate axes, thereby eliminating the use of a lead screw with recirculating balls. In such systems the table once again rides upon a first set of rails in the X-direction, for example, and a second set of rails in the Y-direction. While the first set of rails supports the table and a first linear motor, the second set of rails supports the first set of rafts, the table, the first linear motor, and a second linear motor. The rails slidably support their respective loads upon roller or ball beatings.
In these prior art systems, the second linear motor must drive the weight of the first set of rails and the entire first linear motor along with the table. Once again, rates of acceleration are compromised. The mass necessitates the use of high power linear motors to acceptably accelerate the table to required speeds. Furthermore, the mass limits the rate at which changes in direction may be implemented.
The conventional linear motors employed in positioning devices of the prior art comprise coil assemblies mounted upon a first member, magnet assemblies mounted upon a second member, and the first and second members engaging each other so as to allow linear movement in a single axis. Generally, one member takes the form of a pair of rafts or a channel while the other member slides upon the rails or in the channel by means of ball or roller bearings. In such systems the motion generated is restricted to a single axis. While the table may move linearly in the single axis its orientation remains constant; the table cannot rotate. Furthermore, the rails or the channels must be precisely machined and are subject to wear, thus increasing production and maintenance costs. Finally, if motion is required in a plane rather than in a single axis, an entire second linear motor assembly is employed to move the first linear motor assembly in a direction normal to its axis of motion. This further exacerbates the costs involved.
In the prior art, such as that disclosed in U.S. Pat. No. 3,376,578, positioning devices employ shuttles which float over a surface and are driven by linear stepper motors. Such devices do not provide for rotational motion of the shuttle. Also, the motion such devices are capable of is limited to movement in discrete increments defined the stepper motor controller, the configuration of the stepper motor poles, and the surface configuration of a platen upon which the shuttle rests. Thus, continuously variable positioning cannot be achieved. Another difficulty encountered in such devices is the discontinuous torque applied by the stepping action, which translates into pulsing acceleration and movement; Smooth uniform motion cannot be achieved. Additionally, although pairs of linear stepper motors are employed in such devices, further compensation for offset center of gravities upon the shuttles is not provided nor are means for detecting the torque effects of such offset centers of gravities. Furthermore, such devices are prone to mispositioning of the shuttle due to lost step counts and the inability to independently locate the position of the shuttle.